Stepper motors are widely used in applications in which an accurate positioning of an output shaft is to be provided, their advantage being that the output can be moved to any desired position from a known starting position simply by generating an appropriate number of pulses to drive the shaft incrementally. Stepper motors can thus be operated in an open loop configuration and so there can be a great cost advantage over servo-systems since neither a positional sensor nor feedback control system is required.
The precise mechanical constructions of stepper motors are many and various, but generally a multiple pole motor together with a multi-phase stator winding is employed. The greater the number of poles and phases, the greater the resolution in the positional increment which may be achieved. Multi-phase motors are also capable of high power. Whatever the construction, a stepper motor controller is required to generate, from an applied input pulse, appropriate currents to the motor windings to make the axis of the air gap field step around in coincidence with the input pulse.
In some applications, the movement of the motor may be limited by hard end stops. A mechanical output shaft may be limited to 90.degree. of rotation, for example, the stepper motor shaft may be subject to similar angular limitation or, if the motor drives the output shaft via a gearbox for example, to a maximum number of continuous revolutions. Either way, the motor will be blocked if an attempt is made to drive it past an end stop. Fortunately however, this does not result in damage to the motor since the current demand of the coils does not change between the running condition and the stall condition. Indeed, this property makes the use of stepper motors attractive where loads are to be driven hard against an end stop since no sensing system is required to shut down the currents.
It will be appreciated that due to the differential nature of the position control, stepper motors are not suitable for applications in which absolute positional accuracy is necessary and movement relative to a known reference cannot be guarantied: an application in which the motor may be become blocked for example. Once motion in response to a pulse is blocked, positional accuracy is lost. This can be overcome by fitting a positional or motion sensor at the output but to do so is to loose one of the major advantages of the stepper motor. Hence, use of stepper motors in many applications where they would otherwise be desirable has heretofore been limited.